Im probably being stupid here but Im confused as to when you can use the impulse formula: I = F*t and when you use F*d = 0.5mv^2 for finding a constant applied force.(adsbygoogle = window.adsbygoogle || []).push({});

If a particle's moving at a constant velocity decelerating to 0m/s in a set distance when acted upon by a constant force f, the force I calculate is double when I use I = F*t (momentum lost = force * time of application) than that of when I use

KE lost = Work done = F*d

e.g m= 3Kg u = 4m/s stopping distance(x)= 5m

Using momentum: I = mv - mu = 12Ns

meanwhile v=4 m/s

thereforedx= 4t by integration

ds = 5m

therefore t = 5/4 s

So using I = F*t

F= I/t

F= 12/1.25

F= 9.6N

Using K.E

KE lost = Work done = F*x

F*x = 0.5mv^2

F*x = 0.5*3*4^2

F*x = 24

x=5m

therefore F= 24/5

F= 4.8 N

Why is this half???

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# Homework Help: Change of momentum and kinetic energy

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